linux-kernel - Re: [PATCH, RFC] reimplement flush

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Date:	Mon, 18 Dec 2006 17:00:28 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	David Howells <dhowells@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...l.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH, RFC] reimplement flush_workqueue()

On Tue, 19 Dec 2006 03:43:19 +0300
Oleg Nesterov <oleg@...sign.ru> wrote:

> On 12/18, Andrew Morton wrote:
> >
> > On Mon, 18 Dec 2006 01:34:16 +0300
> > Oleg Nesterov <oleg@...sign.ru> wrote:
> > 
> > > NOTE: I removed 'int cpu' parameter, flush_workqueue() locks/unlocks
> > > workqueue_mutex unconditionally. It may be restored, but I think it
> > > doesn't make much sense, we take the mutex for the very short time,
> > > and the code becomes simpler.
> > > 
> > 
> > Taking workqueue_mutex() unconditionally in flush_workqueue() means
> > that we'll deadlock if a single-threaded workqueue callback handler calls
> > flush_workqueue().
> 
> Well. But flush_workqueue() drops workqueue_mutex before going to sleep ?
> 
> 	flush_workqueue(single_threaded_wq);
> 		...
> 		mutex_lock(&workqueue_mutex);
> 		...
> 		mutex_unlock(&workqueue_mutex);
> 		wait_for_completition();
> 							handler runs,
> 							calls flush_workqueue(),
> 							workqueue_mutex is free

Oh.  OK.  In that case we can switch to preempt_disable() for the
cpu-hotplug holdoff.  Sometime.

> > It's an idiotic thing to do, but I think I spotted a site last week which
> > does this.  scsi?  Not sure..
> 
> Ok, it is time to sleep. I'll look tomorrov and re-send if flush_cpu_workqueue()
> really needs "bool workqueue_mutex_is_locked" parameter.

Hopefully not.
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